fixes jitters by using signals/slots to pass image buffers around

Updating values rapidly would cause strange jitters as a
byproduct of the viewer trying to update from the renderer while
it was still working. Rather than the viewer trying to access the
the renderer, we now send textures in the initial update signal
to keep everything synchronized.

app/render/backend/opengl/openglbackend.cpp

@@ -6,9 +6,7 @@
 #include "functions.h"
 
 OpenGLBackend::OpenGLBackend(QObject *parent) :
-  VideoRenderBackend(parent),
-  push_texture_(nullptr),
-  push_time_(-1)
+  VideoRenderBackend(parent)
 {
 }
 
@@ -58,21 +56,11 @@ void OpenGLBackend::CloseInternal()
 
   //copy_buffer_.Destroy();
   master_texture_ = nullptr;
-  push_texture_ = nullptr;
   //copy_pipeline_ = nullptr;
 }
 
 OpenGLTexturePtr OpenGLBackend::GetCachedFrameAsTexture(const rational &time)
 {
-  if (push_time_ >= 0) {
-    rational temp_push_time = push_time_;
-    push_time_ = -1;
-
-    if (time == temp_push_time) {
-      return push_texture_;
-    }
-  }
-
   const char* cached_frame = GetCachedFrame(time);
 
   if (cached_frame != nullptr) {
@@ -167,11 +155,16 @@ bool OpenGLBackend::TraverseCompiling(Node *n)
   return true;
 }
 
-void OpenGLBackend::ThreadCompletedFrame(NodeDependency path, QByteArray hash)
+bool OpenGLBackend::TimeIsCached(const TimeRange &time)
+{
+  return cache_queue_.contains(time);
+}
+
+void OpenGLBackend::ThreadCompletedFrame(NodeDependency path, QByteArray hash, QVariant value)
 {
   caching_ = false;
 
-  OpenGLTexturePtr texture = path.node()->get_cached_value(path.range()).value<OpenGLTexturePtr>();
+  OpenGLTexturePtr texture = value.value<OpenGLTexturePtr>();
 
   if (texture == nullptr) {
     // No frame received, we set hash to an empty
@@ -197,9 +190,9 @@ void OpenGLBackend::ThreadCompletedFrame(NodeDependency path, QByteArray hash)
   }
 
   // Set as push texture
-  push_time_ = path.in();
-  push_texture_ = texture;
-  emit CachedFrameReady(push_time_);
+  if (!TimeIsCached(TimeRange(path.in(), path.in()))) {
+    emit CachedFrameReady(path.in(), value);
+  }
 
   CacheNext();
 }
@@ -208,7 +201,7 @@ void OpenGLBackend::ThreadCompletedDownload(NodeDependency dep, QByteArray hash)
 {
   frame_cache()->SetHash(dep.in(), hash);
 
-  emit CachedFrameReady(dep.in());
+  emit CachedTimeReady(dep.in());
 }
 
 void OpenGLBackend::ThreadSkippedFrame()